With the late harvest, growers are pushing hard to get fall tillage completed to address compaction from both last fall and this spring. In some cases, producers are not even waiting for soil sample collection or fertilizer applications.
Fall tillage will leave the soil surface 1-3 inches higher than before the tillage operation. A sampling depth of 6 inches before tillage becomes 7-9 inches due to the reduced density of the tilled soil. Many soil probe designs will push the soil away rather than collect the soil. Collecting a proper depth soil sample becomes nearly impossible. Some sources recommend driving over the soil surface and taking the samples from the tire tracks, this can lead to too deep of a sample.
If any fertilizer was applied prior to the fall tillage being completed, it is recommended to wait until next spring to collect soil samples.
If a field has been tilled before soil samples have been collected, soil sampling needs to be delayed by a minimum of 6 to 8 weeks to allow the soil to settle to a density that can be accurately sampled. The time needed for soils to settle can increase under deep or aggressive fall tillage operations. It is best to wait until next spring to collect samples. Incorrect sampling depth will bias a soil sample much more than the seasonal variability between spring and fall soil samples.